The widespread use of narrow and spread spectrum signals modulated by different criteria in aircraft necessitates the use of an adaptive microwave test set. Such an adaptive rf/microwave test set could test a wide variety of communications, radio navigation and electronic warfare equipment.
Traditionally, the parameters evaluation and characterization of amplitude modulated (AM), frequency modulated (FM) and phase modulated (.phi.M) signals are achieved utilizing analog test and measurement instruments. The advancement of high speed analog-to-digital converters and digital signal processing techniques have made a digital AM/FM/.phi.M demodulator an accurate, repeatable and cost effective alternative for the test and measurement of various modulation waveforms. In the test and measurement of electronic warfare (EW), electronic countermeasures (ECM) and communication systems, the message signals which are transmitted from or received by the unit under test (UUT) are often modulated. For instance, signals generated by an airborne radar system are amplitude modulated. Doppler navigators or doppler frequency jammers receive or transmit frequency modulated signals. In communication systems, in order to reduce noise and interference on the message signal and to ease radiation and frequency channel assignments for broadcasting, amplitude or frequency modulation techniques are commonly used. Performance evaluation and characteristic measurements of these systems have conventionally been done by setting up multiple test procedures and employing many specialized instruments. The carrier frequency can be measured using a frequency counter while the modulating signal source is properly terminated or disconnected. Without modulating a carrier signal, the modulating signal (message signal) is analyzed with an oscilloscope or other computer based instrumentation. Finally, the modulator characteristics are measured with a modualtion analyzer and a spectrum analyzer. These testing steps assume that the system can be disassembled and that the individual functional units (or UUTs) are accessible by test engineers. Unfortunately, this assumption may not be valid, especially at remote depot areas where skilled test technicians and sophisticated test and measurement instrumentation are not available.
This invention is directed to a new technique which analyzes modulation parameters, carrier frequency and modulating signal characteristics without requiring reconfiguration or disassociation of the system. It is contemplated that this technique is in compliance with the emerging demand for a new generation of integrated test systems.
Hardward-based Analog Demodulation techniques for AM/FM/.phi.M (amplitude demodulated/frequency modulated/phase modulated) signals have been developed and used for many years. These techniques may be found in K. S. Shanmugam, "Digital and Analog Communication Systems", John Wiley & Sons, New York, N.Y. 10016 (1979); and H. E. Rowe, "Signals and Noise In Communications Systems", D. Van Nostrand Company, Princeton, N.J. (1965).
In order to accurately measure the carrier signal, the modulating waveform and the modulation characteristics, precise analog components are required to construct the demodulator. For instance, in the FM demodulation case, a wide range linear frequency discriminator is necessary to demodulate a modulation signal with a large peak frequency deviation. Because of analog components are nonlinear and temperature dependent, compensation circuits to eliminate harmonic distortions and temperature drift are needed. This will complicate the design and reduce measurement repeatability. Additional instruments such as oscilloscopes or spectrum analyzers are used to measure the demodulating signal. The test and measurement system becomes expensive and less portable.
Equipment, such as radio sets or Doppler frequency jammers, requires fast, accurate and portable testers for the measurement of modulated waveforms. To meet these requirement it was necessary to develop efficient time domain digital AM, FM and .phi.M demodulation techniques. Such demodulation techniques would enable an economical, accurate and portable implementation for the measurement of the modulating signal and the frequency of the carrier signal. It was further desired to process batch data as well as continuous data. The advancement of digital techniques has made waveform measurement and signal parameters characterization accurate, faster and more repeatable than ever before.